The present invention relates to systems and methods for supplying ultra-high-purity ammonia for semiconductor manufacture.
Contamination in IC Manufacturing
Contamination is generally an overwhelmingly important concern in integrated circuit manufacturing. A large fraction of the steps in modern integrated circuit manufacturing are cleanup steps of one kind or another; such cleanup steps may need to remove organic contaminants, metallic contaminants, photoresist (or inorganic residues thereof), byproducts of etching, native oxides, etc.
As of 1995 the cost of a new front end (integrated circuit wafer fabrication facility) is typically more than a billion dollars ($1,000,000,000), and a large fraction of this cost is directed to measures for particulate control, cleanup, and contamination control.
One important source of contamination is impurities in the process chemicals. Since the cleanups are so frequent and so critical, contamination due to cleanup chemistry is very undesirable.
Ammonia Purification
Ammonia (NH.sub.3) is an important process chemical in semiconductor manufacturing. It is very commonly used for deposition of silicon nitride, and may also be used for nitridation or for deposition of other nitrides. Ammonia (in the form of ammonium hydroxide) is also widely used in the basic cleanup portion of the standard RCA cleanup. (The RCA cleanup includes: 1) solvent wash to remove gross organics--in tetrachloroethylene or comparable solvent; 2) basic cleanup--NH.sub.4 OH+H.sub.2 O.sub.2 +H.sub.2 O in proportions of 1:1:5 or 1:2:7; and 3) acid cleanup--HCl+H.sub.2 O.sub.2 +H.sub.2 O in proportions of 1:1:6 or 1:2:8.) See W. Runyan and K. Bean, SEMICONDUCTOR INTEGRATED CIRCUIT PROCESSING TECHNOLOGY (1990), which is hereby incorporated by reference. For semiconductor manufacturing, such cleanup reagents are normally bought as packaged containers. However, this implies that some handling of the solutions in those containers will be necessary, both at the manufacturer's plant and at the use location. Such handling of ultra-high purity chemicals is always undesirable.
Ammonia presents particular difficulties, since liquid ammonia contains both solid and volatile impurities, many of which are damaging to electronic components if present during the manufacturing process. The impurities level and content may vary widely depending on the source as well as the handling method, and all such impurities must be removed before the ammonia can be used in electronic component production lines.
To meet this standard, production facilities have had to obtain high-quality ammonia at considerable cost from the limited sources which are able to supply ammonia at an acceptable grade. Only qualified suppliers can be used, and new suppliers must be qualified before their product can be accepted. This cost and the lack of flexibility add considerably to the cost of the components.
A further constraint is presented by Department of Transportation regulations, under which aqueous ammonia must be shipped at ammonia concentrations no higher than 30%.
Clearly there is a need for a reliable means of supplying ammonia at a purity level which will produce a high yield of acceptable product in ultra-high precision components, and which can meet the requirements of advancing electronics technology.
Ammonia Purification
It has now been discovered that ammonia can be supplied to a production line for high-precision electronic devices in ultra-high purity form by use of an on-site system which draws ammonia vapor from a liquid ammonia reservoir, passes the ammonia vapor through a microfiltration filter, and scrubs the filtered vapor with high-pH purified water. The uniqueness of this discovery is that it can convert commercial grade ammonia to ammonia of sufficiently high purity for high-precision manufacturing without the need for conventional column distillation. The drawing of the ammonia vapor from the supply reservoir serves by itself as a single-stage distillation, eliminating nonvolatile and high-boiling impurities, such as alkali and alkaline earth metal oxides, carbonates and hydrides, transition metal halides and hydrides, and high-boiling hydrocarbons and halocarbons. The reactive volatile impurities that could be found in commercial grade ammonia, such as certain transition metal halides, Group III metal hydrides and halides, certain Group IV hydrides and halides, and halogens, previously thought to require distillation for removal, are now discovered to be capable of removal by scrubbing to a degree which is adequate for high-precision operations. This is a highly unusual discovery, since scrubber technology is traditionally used for the removal of macro-scale, rather than micro-scale, impurities. In the present invention, the scrubber lowers the levels of impurities which are damaging to semiconductor wafer manufacture to less than 1 ppb per element or less than 30 ppb total. For operations where even greater purity is desired, distillation may also be performed subsequent to the scrubbing. An advantage of the invention, however, is that if distillation is included, the scrubber considerably lessens the burden on, and design requirements for, the distillation column, enhancing the product purity even further. The removal of impurities which are close-boiling relative to ammonia, such as reactive hydrides, fluorides, and chlorides, simplifies the distillation column design considerably.
On-Site Preparation of Ultrapure Mixed Cleanup Solutions
The present application discloses preparation of mixed cleanup solutions, such as the RCA acidic cleanup and the RCA basic cleanup, at the site of a wafer fabrication facility, from ingredients which themselves have been ultrapurified at the same site.
The RCA cleanup includes: 1) solvent wash to remove gross organics--in tetrachloroethylene or comparable solvent; 2) basic cleanup--NH.sub.4 OH+H.sub.2 O.sub.2 +H.sub.2 O; and 3) acid cleanup--HCl+H.sub.2 O.sub.2 +H.sub.2 O.) See W. Runyan and K. Bean, SEMICONDUCTOR INTEGRATED CIRCUIT PROCESSING TECHNOLOGY (1990), which is hereby incorporated by reference.
The RCA basic cleanup solution is typically NH.sub.4 OH+H.sub.2 O.sub.2 +H.sub.2 O in proportions of 1:1:5 or 1:2:7. According to one of the innovative teachings disclosed herein, RCA basic cleanup (or analogous cleanup solutions) is generated at the site of a wafer manufacturing plant, by combination of ultra-pure ammonia which has been purified on-site with ultra-pure hydrogen peroxide which has been purified on-site. Thus purity is increased, and the risk of undetected accidental contamination is reduced.